The composite principle states that the sum of the components in a composite can extend the physical characteristics beyond that of each individual component. This principle may be applied to ceramic reinforced, resin matrix composites to exploit the mechanical properties of the ceramic filler when used to reinforce the adaptability and setting characteristics of the resin matrix. A unique aspect of this synergy relies on the transference of the mechanical strains, loads, and the like exerted on the composite, through the resin matrix and into the ceramic filler. This transference relies upon specific mechanisms whereby the filler and resin matrix are combined and bonded physically and/or chemically. The specific properties of interest of the filler may be mechanically, biologically and/or physiologically useful properties. A challenge exists in chemically bonding the typically ceramic inorganic filler with the typically resinous organic matrix. Coupling agents are typically used in this regard and have been moderately successful in bonding the inorganic filler to the organic resin matrix.
The coupling agent plays a role in the transference of properties from the filler to the matrix. The composite properties are influenced by the composition and amount of filler and largely by the degree of coupling or chemical crosslinking at the interface of the ceramic filler and the resin matrix. More specifically, the ability to prepare any ceramic filler (typically silicates) for coupling to an acrylic matrix (DUDMA, BisGMA, TEGDMA, PMMA etc.) is a function of the selected coupling agents (i.e. silanes, siloxanes). The silane coupling process traditionally involves the acid peptisation in aqueous media of the silica bonding functionality of the silane by creating an Si—O linkage that will bond to the acidified surface of the ceramic filler. The acrylic functionality of the coupling agent plays a role in terms of adherence to the ceramic filler, while maintaining the functionality of the adjacent acrylic double-bond, that upon mixture with the resin matrix will have the ability to chemically crosslink therewith.
The above mechanism is relatively straightforward and predictable when the filler is “inert” or non-reactive. However, if a filler is reactive in a manner that makes the acid peptized linkage difficult and impractical, and the reaction of the filler affects the acrylic functionality, then the coupling with a standard silane becomes problematic for particular fillers. One such group of fillers of particular utility includes those glasses that may be described as bioactive or bio-reactive. Typically, bioactive glasses react in the body through an initial ion exchange mechanism to impart bonding to surrounding bone tissue. As these glasses are inherently reactive, the physical and chemical character of the exposed surfaces tend to be in flux under the conditions surrounding the traditional silanation process, making these glass surfaces ‘moving targets’ for silanation. Accordingly, the incorporation of said bioactive fillers into resin matrices has been met with difficulty in achieving appropriate coupling with such reactive alkali containing fillers.
Alkali releasing fillers represent a challenge to the traditional silanation method, the effectiveness of the acrylic groups and the eventual inclusion into a composite. Alkali that has not been well coupled will lead to poor composite properties, instability from both a mechanical and chemical standpoint. Alkali can lead to premature depletion of available amine used in crosslinking, leading to a setting composite that does not set, or to the creation of peroxide free radicals that lead to the premature autopolymerization of the composite. Neither is acceptable for stable self-setting composites. Silanation of such materials is currently done by applying a sufficiently thick coat of silane material over the reactive ceramic particles such that the silane coupling agent is adhered thereto through a physical caking mechanism, and may almost be thought of as a secondary matrix in and of itself. Thus, there remains a need for a process for applying a thin and even coating of silane coupling agent to reactive ceramic filler particles prior to their incorporation into a resinous matrix. The present invention addresses this need.